1. FIELD OF THE INVENTION
The preparation of 2,2-dihydrocarbyl-3-propiolactones by thermolysis of oligomeric 3-hydroxy-2,2-dihydrocarbylpropionic acid that contains a carboxyl end, in the presence of selected cationic catalysts is disclosed.
2. TECHNICAL BACKGROUND
The general concept of heating the polyesters of hydroxy acids to prepare lactones, and particularly caprolactones (forming seven-membered rings) has been known for a long time, see for example W. H. Carothers, et. al., J. Am. Chem. Soc., vol. 54, p. 761-772 (1932) and references therein. A more recent reference is, for example, U.S. Pat. No. 4,550,181, in which polycaprolactone is heated in the presence of water and a hydrogen halide to form caprolactone. However, preparation of lactones with four ring atoms, 3-propiolactones, is more difficult by these methods because of the propiolactone's greater tendency to undergo thermal degradation [see for example T. L. James and C. A. Wellington, J. Am. Chem. Soc., vol. 91, p. 7743-7746 (1969)].
Japanese Patent Application 48-30067 describes the preparation of episilon-caprolactone by the thermolysis of a polycaprolactone oligomer having a degree of polymerization of 2 to about 20. However, the process is carried out with the ester of the oligomeric caprolactone or in the presence of an alcohol. The application states that it is preferable "that the carboxyl groups derived from these free acids be esterified" (from the translation). Thus free carboxyl groups are undesirable according to this Japanese Application.
U.S. Pat. No. 3,751,435 describes the thermolysis above 245.degree. C. of poly(pivalolactone) in the presence of certain basic substances or neutral salts to give pivalolactone (2,2-dimethyl-3-propiolactone). No mention is made of the degree of polymerization of the poly(pivalolactone) used or the end groups it contains.
I. Luderwald, Makromol. Chem., vol. 182, p. 867-871 (1981), reports the thermolysis of poly(pivalolactone) in the presence of certain metal salts. No mention is made of the original degree of polymerization, or the end groups of the poly(pivalolactone) used.
L. E. Manring, et. al., Macromolecules, vol. 23, p. 1902-1907 (1990), reports that oligomers of poly(2-methyl-2-n-propyl-3-propiolactone) containing carboxyl ends, in the presence of certain cations, thermolyze faster than higher molecular weight polymers, or polymers that do not contain carboxyl ends.
It is an object of this invention to provide a method for the preparation of 2,2-dihydrocarbyl-3-propiolactones by the thermolysis of oligomers of the corresponding hydroxy acids having a relatively low degree of polymerization, and which contain carboxyl(ate) ends, in the presence of certain cations. This combination of conditions allows the use of lower temperatures and/or shorter residence times for the thermolysis, which in turn results in superior yields of the desired propiolactone.